Hi everyone,

my 97’ Jaguar X300 4.0 is currently consuming about 18–19 liters per 100 km (that’s roughly 12–13 mpg US) on short trips.

Do you have any idea what could be causing this? What are common issues that lead to such high fuel consumption?

Is there some kind of checklist or step-by-step guide I could follow to systematically track down the problem?

Thanks in advance for any advice!

 

In general, the engine runs smoothly; I haven’t noticed anything unusual there.

However, it’s a bit harder to start the first time — it cranks a few more times than usual before firing up.

Once the car has been started once, it starts normally afterwards.

But the fuel consumption is just way too high.

 

MAF, bad coils, dirty/bad injectors, bad 02 sensors.
how long is a piece of string situation. been down this road myself. got it sorted but hard to say exactly what was wrong. with 370000km on my car i suspect it was a combination of many things

 

I'm kind of in the same boat but I get between 15 to 16 mpg if I don't have a lot of highway miles. I've replaced coils, plugs, o2 sensors and fuel filter and it hasn't really changed consumption much. No CEL since replacing the O2 sensors. The only performance is I note is slow acceleration when stepping on it to get on the highway. My 0-60 time is measured with a sundial.

 

That’s my concern — replacing everything now and it doesn’t make any improvement, or the error is still there.

 

The engine coolant sensor at 160 F triggers the engine ECU into closed loop which only then uses the O2 sensors for efficient fuel trim / mixture , Short Term Fuel Trim target 0.0 but not perfect in real world

In the starting sequence ...................................

Camshaft not Crankshaft CEL code P0340 vs 335 Crankshaft

As the first rotations of the engine the camshaft position sensor is used until the crankshaft position sensor takes over , the engine will start with the CMPS connector removed

The CMPD sensor has targets for each 6 TDC for the cylinders Vs. 1 TDC on the CKPS which aids starting faster , My CMPS connector was dirty , A good CMPS reads 600 ohms as a basic meter reading

 

I’d start with a cheap code reader and see if the O2 sensors need replacement.
With my 3.2 it was a combination of these failing plus a default new engine temperature sensor. (Gave correct readings to ECU til nearly at operating temperature then dropped to almost zero)

So mixture consistently too rich- but ran very smoothly.

 

I have an OBD2-tool and it only shows me error code P1621. I don't think it has anything to do with the very high fuel consumption. But I'm not sure.
I'll take a short drive later and record the OBD2 data while I'm doing so. Let's see what the values say then.

 

I wasn’t getting an error message but the graphs from each 02 sensors were very different.
While since I did it and can’t rem which output I was using .

Best of luck .

 

The P1621 code if a recall correctly is an odd code that shows up that some have had and contributors have not nailed down . See page 110 abouts of the Jaguar 801s doc for the official AJ16 engine codes , there is the earlier inline 6 AJ6 and the V - 12 included in this doc

www.jagrepair.com


The O2 sensors are different Titania based sensors ( they operate totally differently ) that may not be picked up on your live data reader ( looking for the more common ) and the end result of the sensors being active and correct is the ECU / engine resulting in the short term fuel trim target 0.0 %

The O2 sensors even being able to give a correct signal is based on if they are heated up ( the sensors themselves ) to operating temp of X ( sensor physics ) , I question the X300 live data graph below as the O2 sensors readings stay constant

There is one fuse ( heating elements inside )2 sensors ) for all four sensors as right engine bay fuse box # 10 , 12 , 14 , 16 without looking it up , and a connector ( Papa Indy 1 and Papa Indy 61 ) subject to a TSB that this one fuse wire goes through . Wire is in Papa Indy 61 position 12 as White / Green color wire




The accuracy of the O2 sensors return wires is dependent on the wires shield ground ( it is a miny coax cable ) as the small ground studs on the engine rear firewall

I had green corrosion on the rear sensors ( only ones used for engine regulation ) connectors , do not mix up the connector hookup location as they can be swapped and cause issues like maybe yours









This would be red color ECU connector

 

Okay, wow... Thank you very much for the detailed explanation. I'll see how I manage with it as a layperson

I drove a slightly longer distance than usual yesterday. Country roads, light traffic (about 80-100 km/h), engine at operating temperature. After I pressed reset on the onboard computer to restart the average consumption, voilΰ – about 10 liters per 100 km. It seems that once the engine is at operating temperature, it has a normal fuel consumption.

So now I should check two things:

Does the engine take too long to reach operating temperature (the fans starts as soon as the ignition is on - is that how it's supposed to be on a X300? As soon as the ignition is turned off, the fans also stop immediately. So there's no continued running of the fan.), or are the heaters for the lambda sensors not working – right?

 

i is to be expected that cold running has increased fuel consumption.
in cold running mode the idle speed is increased and extra fuel is pumped to get engine up to running temp.
i no longer drive my jag to and from work as it is a 15-20in trip in medium traffic so my fuel consumption will get up to 18l per hundred on occasions on that trip.
when i drive it more normal conditions on longer drives it will sit 13-14l/100.
full highway driving it will get down to 10l/100.

 

Can you pull the LTFT (Long Term Fuel Trim) from the car using an OBD2 reader?

 

The fans coming on at the very beginning is normal after a TSB was accomplished

First thing on thermostat is to not install a large O - ring , even if it comes with one , cracks T housing bolt ears

The sensor connector can be checked with a meter to see if the heating elements are intact , , all 4 sensors should read the same resistance

On a 4 sensor model the front O2 sensors can be swapped with the aft as the aft are the only 2 used for engine regulation

The front sensors if present are easily seen as pointing fwd just down from the exhaust manifold castings

 

Its going to be worth Checking the fuel pressure at the fuel rail if your issues are not yet resolved ( not so easy as there is no additional ports to do this ) my fuel pressure regulator failed and was causing almost identical symptoms to the OP's post.

In the end i welded a port to a second hand fuel rail and fitted a manual gauge so i could witness the fuel pressure . This was a couple years ago and the numbers escape me but it was significantly off. I fitted an aftermarket reg remotely mounted.

There is some really great advice in this thread - but don't get to caught up on logs and trim tables and OBD . Fuel pressure is not reported on these cars so to my knowlage it will not trigger any kind of fault or code.

 

Fuel pressure is set to 43 PSI ?